Currently, a large variety of optical recording mediums are in use. Typical of these are optical discs, such as a CD (Compact Disc) or a DVD (Digital Versatile Disc), and magneto-optical discs. The information signals, recorded on the optical disc of this type, are read out by condensing the light beam, radiated by a light source, provided in the optical pickup device, by an objective lens, illuminating the light beam to a signal recording surface of the optical disc and by detecting the light beam reflected back from the signal recording surface by a photodetector. It is noted that the information signals, recorded on the CD or on the DVD, are read out by detecting changes in reflectivity of the light beam, reflected back from the signal recording surface, by the photodetector. In the case of the magneto-optical disc, the information signals are read out by detecting the Kerr rotation of the light beam reflected from the signal recording surface of the disc.
The information signals recorded on the above-described optical disc are read out at least as the objective lens condensing the light beam radiated from the light source is kept out of contact with the optical disc.
The light spot of the light beam, condensed on the signal recording surface of the optical disc by the objective lens of the optical pickup device, is approximately given by λ/NA, where λ is the wavelength of the illuminated light beam and NA is the numerical aperture. The resolution is also proportionate to this value.
Meanwhile, the following equation holds for NA:NA=n≅sin θwhere n is the refractive index of the medium and θ is the angle of the ambient light incident on the objective lens.
If the medium is air, the value of NA cannot exceed 1 (unity). As a technique for a case in which this limit value is exceeded, an optical pickup device employing a solid immersion lens has been proposed (I. Ichimura et al., ?Near-Field Phase-Change Optical Recording of 1.36 Numerical Aperture≅, Jpn. J. Appl. Phys. vol. 39, 962-967, 2000).
The solid immersion lens is formed of a material of the same refractive index as the optical disc substrate, and is made up by a spherical portion, forming a fraction of a sphere, and a flat portion facing the optical disc surface. The solid immersion lens, used in an optical pickup device, is used as the flat portion kept extremely close to the optical disc surface. The boundary surface between this solid immersion lens and the optical disc is traversed by an evanescent wave. It is this evanescent wave that reaches the signal recording surface of the optical disc.
When the information signals, recorded on the optical disc, are read out using this optical pickup device, a suitable gap needs to be provided between the rotationally driven optical disc and the solid immersion lens forming the objective lens. Since the gap as the air layer is provided between the optical disc and the solid immersion lens, an evanescent wave must be used in order to make the numerical aperture (NA) of the objective lens larger than 1 (unity). The evanescent wave is attenuated exponentially as from the boundary surface. It is therefore necessary that the gap between the optical disc and the solid immersion lens shall be of an extremely small value on the order of one-tenth of the light emission wavelength λ of the light source provided on the optical pickup device, while it is necessary that the solid immersion lens shall be close to the signal recording surface.
For controlling the gap in this manner, a servo method has so far been proposed in which an electrode is formed on the surface of the solid immersion lens, the capacitance across the electrode and the optical disc is detected to derive a gap error signal, and in which the distance between the solid immersion lens and the optical disc is controlled based on this error signal.
For implementing this method, it is necessary to form the electrode on the surface of the solid immersion lens and to take out a signal line from this electrode to a controlling circuit. This, however, complicates the apparatus to render the manufacture of the optical pickup device difficult.
On the other hand, the present Applicant has already proposed, in the specification and drawings of the Japanese patent Application No. H10-249880, a method for detecting the return light from a glass master disc and for using thus return light as a gap error signal.
This method exploits the phenomenon in which, if the gap between the solid immersion lens and the glass master disc is zero, the solid immersion lens surface is contacted with a transparent photoresist on the glass master disc, so that there occurs no light reflection from the lens surface, however, if the gap is not zero, the light totally reflected on the solid immersion lens surface is returned, this light being used for detecting the gap.
This method can be used when the glass master disc is used and the photoresist for light exposure is transparent. This method, however, cannot be used for a case in which a reflective film, such as an aluminum film, a phase change film or a photomagnetic recording film, is formed on the disc surface, as in an optical disc, because the light then undergoes reflection on the optical disc surface, even though the gap between the disc and the solid immersion lens is zero.